The present disclosure relates generally to detecting neutrons and, more particularly, to locating neutron emitting sources.
Special Nuclear Material (SNM) is defined as the type of material that can be used to fabricate a nuclear weapon. Detection of this type of material with a sensor is a challenge due to a variety of false alarms. Many of the false alarms can be due to natural sources common in many locations and therefore difficult to avoid. Additionally, many of the false alarms are due to the phenomenology of the sensor design.
One way to detect SNM is to detect emitted gamma radiation, which comes from SNM sources. However, a large natural background of gamma radiation from cosmic sources and terrestrial sources, such as potassium-40 and Iron-55, can cause false alarms. In addition, the SNM may be shielded to greatly reduce emission of associated gamma radiation and, thus, reduce the likelihood of detection.
Alternatively, SNM may be detected by detecting neutrons emitted from the SNM. Various processes such as nuclear absorption and excitation exist for neutron detectors to detect neutrons. In the nuclear absorption process, a neutron is absorbed by the nucleus of some atom in a detector. This daughter nucleus will then decay emitting decay products such as charged particles or gamma radiation, which are subsequently detected. While this process can detect thermal neutrons, it may not be efficient at detecting fast neutrons emitted from SNM. Additionally, it may be desirable to detect fast neutrons emitted by contamination or radioactive leaks at nuclear power facilities or facilities handling nuclear materials.
In the nuclear excitation process, an incoming neutron will scatter from a nucleus of some atom in the detector. This moves this nucleus to an excited energy state from which it will return to a base state by emitting gamma radiation, which is then detected. These types of detectors have an inherent problem in that they are very good gamma ray detectors. They will detect noise such as typically all cosmic background radiation and all natural background gamma rays.
In addition to the above deficiencies, these types of neutron detectors typically do not provide any directional information leading to a point in space from where the detected neutrons came. Some types of neutron detectors used for homeland security purposes do not lend themselves to mobility, but are generally permanently installed or difficult to move. Hence, improvements in neutron detection technology and, particularly, in portable neutron detectors would be well appreciated in the homeland security industry as well as others.